D. Rodeghiero et al/ Materials Science and Engineering 4244(1998)11-21 Nia-Al,O 3 20 enforcement content(vol %) Fig. 6. Fracture toughness data of the sol-gel derived composites as a function of reinforcement content. a backscattered electron SEM micrograph of a hot- ing approaches. Indeed, such agglomerates have been pressed 50/50 vol. Fe/a-AL, O, composite is shown in shown to be detrimental to the mechanical properties Fig. 4. As would be expected at higher metal contents, (particularly the fracture strength) of Sic-reinforced the metallic phase of this composite appears continuous ceramics[28]. Finally, virtually all of the whiskers in the and somewhat coarser. This continuity is confirmed by micrographs shown here are seen to be intact and the resistivity experiments which find the material to be undamaged, with good bonding between whisker and conducting. Furthermore, the transition from insulating matrix to conducting behavior for all of the metal-ceramic The density, relative density and Youngs modulus of composites investigated in this work occurs between a complete series of Ni/a-Al2O3 metal-ceramic com- metal fractions of 15 and 20 vol % This range agrees posites are listed in Table 1. Literature values for pure extremely well with percolation theory which predicts Ni and pure a-Al2O3 are also included for reference. As hree-dimensional continuity in random systems at would be expected, the density of the composites near a volume fraction of 17%[27]. However, the most creases with Ni content. More importantly, the relative important features of Fig. 4 are the extremely high densities of the cermets are all high, indicating an dispersion and uniformity which are present despite the effective sintering process. The Youngs modulus values now rather large metal volume fraction vary steadily between the pure Ni and a-Al2O3 bounds Fortunately, the excellent microstructures provided However, these numbers are somewhat less than those by the sol-gel techniques used in this work are not predicted by theoretical constructs such as the Hashin limited to the metal-ceramic composites. Fig. 5(a) Shtrikman upper and lower bounds to the Halpin-Tsai shows an optical micrograph of a hot-pressed 20 /80 model [31]. This is due to the effects of porosity on the vol. SiC(whisker)/a-Al,O, ceramic-ceramic com- Youngs modulus of the materials. Phani et al. studied posite. Since the view shown is of a pellet face perpen- several different materials and arrived at the following dicular to the hot-pressing direction, most of the Sic equation for the relationship between porosity and whiskers present are seen to lie flat; however, some elastic modulus equiaxed features due to whiskers perpendicular or at inclined angles to this pellet face are also visible. Fig E=Eo[l-PPn+I 5(b)shows an optical micrograph of the same com- where E is the actual Youngs modulus with porosity posite but of a face parallel to the hot-pressing direc- present, Eo is the theoretical modulus or modulus with- tion. As a result, most whiskers intersect the plane of out porosity, the quantity [1-Pl is the relative density the picture. Furthermore, the few whiskers which do lie (P is the percentage porosity), and the exponent n is an in the plane of the micrograph are oriented horizontally empirical constant commonly taken as 1 [32]. By virtue expected due to the hot-pressing direction in of Eq.(2), Table I also lists the Youngs modulus mple. The primary feature to notice in these figures values the Ni/a-Al2O3 composites would have if the lack of Sic whisker agglomerates which are a very porosity were present(Eo). These values are larger than common and serious problem when using powder mix the actual modulus values but more in line with theo-18 E.D. Rodeghiero et al. / Materials Science and Engineering A244 (1998) 11–21 Fig. 6. Fracture toughness data of the sol–gel derived composites as a function of reinforcement content. A backscattered electron SEM micrograph of a hot￾pressed 50/50 vol.% Fe/a-Al2O3 composite is shown in Fig. 4. As would be expected at higher metal contents, the metallic phase of this composite appears continuous and somewhat coarser. This continuity is confirmed by the resistivity experiments which find the material to be conducting. Furthermore, the transition from insulating to conducting behavior for all of the metal–ceramic composites investigated in this work occurs between metal fractions of 15 and 20 vol.%. This range agrees extremely well with percolation theory which predicts three-dimensional continuity in random systems at or near a volume fraction of 17% [27]. However, the most important features of Fig. 4 are the extremely high dispersion and uniformity which are present despite the now rather large metal volume fraction. Fortunately, the excellent microstructures provided by the sol–gel techniques used in this work are not limited to the metal–ceramic composites. Fig. 5(a) shows an optical micrograph of a hot-pressed 20/80 vol.% SiC(whisker)/a-Al2O3 ceramic–ceramic com￾posite. Since the view shown is of a pellet face perpen￾dicular to the hot-pressing direction, most of the SiC whiskers present are seen to lie flat; however, some equiaxed features due to whiskers perpendicular or at inclined angles to this pellet face are also visible. Fig. 5(b) shows an optical micrograph of the same com￾posite but of a face parallel to the hot-pressing direc￾tion. As a result, most whiskers intersect the plane of the picture. Furthermore, the few whiskers which do lie in the plane of the micrograph are oriented horizontally as expected due to the hot-pressing direction in the sample. The primary feature to notice in these figures is the lack of SiC whisker agglomerates which are a very common and serious problem when using powder mix￾ing approaches. Indeed, such agglomerates have been shown to be detrimental to the mechanical properties (particularly the fracture strength) of SiC-reinforced ceramics [28]. Finally, virtually all of the whiskers in the micrographs shown here are seen to be intact and undamaged, with good bonding between whisker and matrix. The density, relative density and Young’s modulus of a complete series of Ni/a-Al2O3 metal–ceramic com￾posites are listed in Table 1. Literature values for pure Ni and pure a-Al2O3 are also included for reference. As would be expected, the density of the composites in￾creases with Ni content. More importantly, the relative densities of the cermets are all high, indicating an effective sintering process. The Young’s modulus values vary steadily between the pure Ni and a-Al2O3 bounds. However, these numbers are somewhat less than those predicted by theoretical constructs such as the Hashin– Shtrikman upper and lower bounds to the Halpin–Tsai model [31]. This is due to the effects of porosity on the Young’s modulus of the materials. Phani et al. studied several different materials and arrived at the following equation for the relationship between porosity and elastic modulus: E=E0 . [1−P] 2n+1 (2) where E is the actual Young’s modulus with porosity present, E0 is the theoretical modulus or modulus with￾out porosity, the quantity [1−P] is the relative density (P is the percentage porosity), and the exponent n is an empirical constant commonly taken as 1 [32]. By virtue of Eq. (2), Table 1 also lists the Young’s modulus values the Ni/a-Al2O3 composites would have if no porosity were present (E0). These values are larger than the actual modulus values but more in line with theo-